Fuel assembly

ABSTRACT

A fuel assembly for use in a core of a nuclear power reactor. The assembly includes a plurality of helically twisted fuel elements supported by a frame in a fuel rod bundle. Each of the fuel elements includes fissile material. When viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle define a substantially circular perimeter. The fuel elements are arranged in a mixed grid pattern that includes a first, rectangular grid pattern and a second, triangular grid pattern.

CROSS-REFERENCE

This application is a continuation of U.S. application Ser. No.14/856,084, filed Sep. 16, 2015, which is a non-provisional of U.S.Application No. 62/050,985, filed on Sep. 16, 2014. U.S. applicationSer. No. 14/856,084 is also a continuation-in-part of U.S. applicationSer. No. 14/081,056, filed on Nov. 15, 2013, now U.S. Pat. No.10,170,207, which claims priority to U.S. Provisional Application No.61/821,918, filed on May 10, 2013. U.S. application Ser. No. 14/856,084is also a continuation-in-part of U.S. application Ser. No. 13/695,792,filed on Jun. 3, 2013, now U.S. Pat. No. 10,037,823, which is the U.S.National Stage of PCT/US2011/036034, filed on May 11, 2011, which inturn claims priority to U.S. Application No. 61/444,990, filed Feb. 21,2011, U.S. Application No. 61/393,499, filed Oct. 15, 2010, and U.S.Application No. 61/333,467, filed May 11, 2010. The entire content ofall of the foregoing applications is expressly incorporated herein byreference.

BACKGROUND Technical Field

The present invention relates generally to nuclear reactors and nuclearfuel assemblies used in the core of nuclear reactors. More specifically,the present invention relates to Canadian Deuterium-Uranium (CANDU)heavy-water reactors, and fuel assemblies for use in the same.

Related Art

FIGS. 1A and 1B depict simplified cross-sectional views of examples ofconventional fuel assemblies 10. FIG. 1A depicts a fuel assembly 10 ofthe PWR type, and FIG. 1B depicts a fuel assembly 10 of the water-cooledwater-moderated power rector (VVER) type. In FIG. 1A, the fuel rodassembly 10 comprises fuel rods assembled into a square grid. The PWRfuel assembly 10 of FIG. 1A has fuel rod bundle self-spacing that can bedescribed as having a square cross-sectional shape. In FIG. 1B, the fuelassembly 10 comprises fuel rods arranged into a triangular grid. TheVVER fuel assembly 10 of FIG. 1B has fuel rod bundle self-spacing thatcan be described as having a regular hexagonal cross-section shape.

When these assemblies are fitted into a tube 12, empty segments not usedby the fuel rod assembly are formed, as shown by the shaded area 14located between the tube 12 and the square 14 in FIG. 1A, and betweenthe tube 12 and the hexagon 16 in FIG. 1B. According to embodiments, anassembly in a square grid occupies approximately 63.7% of the area ofthe circumscribed circle (e.g., tube 12), while an assembly in atriangular grid occupies approximately 82.7% of the area of thecircumscribed circle (e.g., tube 12).

It is known to use the empty space to address concerns of fuel rod andassembly swelling during burnup. It is also known to fill these areaswith a burnable absorber, etc.

SUMMARY

According to an embodiment, a fuel assembly for use in a core of anuclear power reactor can include a frame shaped and configured fitwithin the nuclear reactor internal core structure; and a plurality ofhelically twisted fuel elements supported by the frame in a fuel rodbundle, with each of the fuel elements comprises fissile material. Asviewed in a cross-section that is perpendicular to an axial direction ofthe fuel assembly, the outermost fuel elements of the fuel rod bundlecan define a substantially circular perimeter (e.g., dodecagon).According to embodiments, the frame can be shaped and configured to fitwithin a pressure tube of a CANDU reactor.

According to embodiments, each of the plurality of fuel elements canhave substantially the same circumscribed diameter. The plurality offuel elements can be arranged in concentric circles. Additionally oralternatively, the plurality of fuel elements can be arranged into amixed grid pattern that includes a first, rectangular grid pattern and asecond, triangular grid pattern.

According to embodiments, the first, rectangular grid pattern and thesecond, triangular grid pattern can at least partially alternate withone another. Some of the plurality of fuel elements can be separatedfrom adjacent fuel elements by a common centerline-to-centerlinedistance, and a circumscribed diameter of some of the plurality of fuelelements can equal the centerline-to-centerline distance.

According to embodiments, each of the fuel elements can have amulti-lobed profile that includes ribs, for example, spiral ribs. Theribs of adjacent fuel elements can periodically contact one another overthe axial length of the fuel elements to at least partially maintain thespacing of the fuel elements relative to each other. According toembodiments, the fuel elements can comprise extruded fuel elements.

According to embodiments, the plurality of fuel elements can consist of61 fuel elements.

According to embodiments, the frame can include a structurecircumscribing the fuel rod bundle, such that all of the fuel elementsare located inside the structure. The structure can comprise a shroud.When viewed in a cross-section that is perpendicular to an axialdirection of the fuel assembly, the shroud can define a cross-sectionsubstantially defining a circle or dodecagon. When viewed in across-section that is perpendicular to an axial direction of the fuelassembly, the fuel assembly can occupy greater than about 64%, morespecifically greater than about 83% of the internal cross-sectional areaof a tube circumscribing the fuel assembly. According to an embodiment,the fuel assembly can occupy between about 83% and about 95% of theinternal cross-sectional area of the tube circumscribing the fuelassembly.

According to embodiments, the fuel assembly is thermodynamicallydesigned and physically shaped for operation in a conventionalland-based nuclear power reactor of a conventional nuclear power planthaving a reactor design that was in actual use before 2014, and theframe is shaped and configured to fit into the land-based nuclear powerreactor in place of a conventional fuel assembly for said reactor. Forexample, the conventional land-based nuclear power reactor can be aCANDU reactor.

According to another aspect of the present invention, a nuclear reactorincludes a core and one or more fuel assemblies disposed within thecore. The fuel assembly can include: a frame shaped and configured tofit within the core; and a plurality of helically twisted fuel elementssupported by the frame in a fuel rod bundle, with each of the fuelelements comprising fissile material. As viewed in a cross-section thatis perpendicular to an axial direction of the fuel assembly, theoutermost fuel elements of the fuel rod bundle can define asubstantially circular perimeter. According to embodiments, the nuclearreactor is a CANDU reactor comprising pressure tubes, and the frame isshaped and configured to fit within the pressure tubes.

According to embodiments, each of the plurality of fuel elements canhave substantially the same circumscribed diameter. The plurality offuel elements can be arranged in concentric circles, and/or theplurality of fuel elements can be arranged into a mixed grid patternthat includes a first, rectangular grid pattern and a second, triangulargrid pattern. The first, rectangular grid pattern and the second,triangular grid pattern can at least partially alternate with oneanother.

According to embodiments, the nuclear reactor was in actual use before2014.

According to embodiments, each of the fuel elements has a multi-lobedprofile that includes spiral ribs. The ribs of adjacent fuel elementscan periodically contact one another over the axial length of the fuelelements to at least partially maintain the spacing of the fuel elementsrelative to each other. According to embodiments, the fuel elements cancomprise extruded fuel elements.

According to embodiments, the frame of the fuel element comprises astructure circumscribing the fuel rod bundle, such that all of the fuelelements are located inside the structure. The structure can comprise ashroud that when viewed in a cross-section that is perpendicular to anaxial direction of the fuel assembly, defines a cross-sectionsubstantially defining a circle or dodecagon.

These and other aspects of various embodiments of the present invention,as well as the methods of operation and functions of the relatedelements of structure and the combination of parts and economies ofmanufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. In one embodiment of the invention, the structuralcomponents illustrated herein are drawn to scale. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention. In addition, it should be appreciatedthat structural features shown or described in any one embodiment hereincan be used in other embodiments as well. As used in the specificationand in the claims, the singular form of “a,” “an,” and “the” includeplural referents unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the present invention, aswell as other features thereof, reference is made to the followingdescription which is to be used in conjunction with the followingdrawings, wherein:

FIG. 1A is a simplified cross-sectional view of a conventional fuelassembly having fuel rods assembled in a square grid;

FIG. 1B is a simplified cross-sectional view of a conventional fuelassembly having fuel rods assembled in a triangular grid;

FIG. 2 is a simplified cross-sectional view of a layout of a self-spacedfuel assembly made up of 61 fuel rods in a square-triangular grid,according to an embodiment;

FIG. 3 is a simplified cross-sectional view of a layout of a self-spacedfuel assembly made up of 19 fuel rods in a square-triangular grid,according to an embodiment;

FIG. 4 depicts a cross-sectional view of an embodiment of a fuelassembly at an initial reference position along the fuel assembly,referred to herein as the initial 0° position;

FIG. 5 depicts a cross-sectional view of the fuel assembly of FIG. 4 ata 30° fuel rod rotation, or at a lengthwise displacement of 1/12 of thefuel rod swirl pitch, with respect to the initial 0° position of FIG. 4;and

FIG. 6 depicts a cross-sectional view of the fuel assembly of FIG. 4 ata 60° fuel rod rotation, or at a lengthwise displacement of ⅙ of thefuel rod swirl pitch, with respect to the initial 0° position of FIG. 4.

DETAILED DESCRIPTION

Embodiments described herein can increase the fuel burnup power and/orlevel (operating time until unloading) of a CANDU fuel assembly and/orreactor as a whole, while maintaining or increasing the level of safety.According to embodiments, this can be achieved through the use of fuelassemblies made from twisted, self-spaced, monolithic fuel rods, forexample, the extruded uranium-zirconium (U—Zr) fuel rods disclosed inapplicant's co-pending U.S. application Ser. Nos. 14/081,056 and13/695,792, the entire contents of which are expressly incorporatedherein by reference.

CANDU fuel assemblies typically utilize very short (e.g., on the orderof 50 cm) fuel rods. Embodiments of the present invention providepartially or fully self-spaced assemblies of CANDU fuel rods. Forexample, some fuel assemblies disclosed herein provide for self-spacingof all the fuel rods among themselves (e.g., rib by rib). However,alternative embodiments can include non-self-spaced arrangements.Embodiments can include a frame having a shroud, or other channel ordevice surrounding all or a part of the fuel rod bundle (referred togenerally herein as a “shroud”), and better utilize the space availableinside the shroud than is possible with the prior art. For example, aswill be described in more detail below, embodiments use a“square-triangular” fuel rod grid in an array.

FIG. 2 is a simplified cross-sectional view of an embodiment of aself-spaced fuel assembly 100. The fuel assembly can include 61 fuelrods 102 in a square-triangular grid, however, other configurations maybe possible. The fuel assembly shown in FIG. 2 can have the same orsimilar envelope as an Advanced CANDU Reactor (ACR) CANDU Flexible(CANFLEX) 43-element assembly. Whereas a typical CANFLEX assembly has 43fuel elements each with an outer diameter of about 13.5 mm, the fuelassembly 100 shown in FIG. 2 can have 61 fuel elements 102 each with anouter diameter of about 11.5 mm, however, other quantities and sizes offuel elements are contemplated.

The fuel assembly of FIG. 2 can be fitted into a shroud 104. Forexample, the shroud 104 can have a cross-section in the shape of adodecagon, however, other shapes are envisioned. According toembodiments, the radius R of a circle circumscribing the fuel elements102 can be less than or equal to 51 mm. According to embodiments, theinner radius of the shroud 104 can be about 51.7 mm, however, otherembodiments are possible. Shroud 104 can have a dodecagon shape, and candefine a width h across the flats of about 100 mm (≤99.99 mm). Accordingto embodiments, the square-triangular grid of 61 fuel elements definesan outer perimeter that occupies approximately 95.5% of the area of thecircumscribed circle (e.g., the shroud 104 or pressure tube). Withreference to FIG. 3, the central area of 19 fuel rods 102 can fit nearlyperfectly into a tube. According to embodiments, the radius R19 of acircle circumscribing the central 19 fuel rods can have a diameter of3.922 mm, however, other dimensions are possible.

Referring to FIGS. 2 and 3, the fuel elements can be located in firstand second grid patterns intermixed with one another to form what isreferred to herein as a “square-triangular grid.” The first grid patternincludes squarely arranged rows and columns of fuel elements having acenterline-to-centerline distance between the rows and columns thatequals the common circumscribed diameter “d” of the fuel elements (seereference 106 in FIG. 3 for an example of the first “square” grid). Thesecond grid pattern includes equilateral triangles in which a length ofeach side of each triangle (i.e., the centerline-to-centerline distancebetween adjacent fuel elements defining the corners of each triangle) isthe common circumscribed diameter “d” of the fuel elements (seereference 108 in FIG. 3 for an example of a second “triangular” grid).Thus, the second/triangular grid pattern 108 is different from thefirst/square grid pattern 106. According to alternative embodiments,additional and/or alternative grid patterns could also be used (e.g.,rectangular grid patterns, isometric grid patterns, parallelogrampatterns, other regular repeating patterns) without deviating from thescope of the present invention. According to embodiments, a given fuelelement 102 may be located in a square grid pattern with one set ofsurrounding fuel elements, and simultaneously be located in a triangulargrid pattern with another set of surrounding fuel elements, however,other configurations are possible.

Still referring to FIGS. 2 and 3, the square 106 and triangular 108 gridpatterns can alternate with one another when viewed from one or moreperspectives. For example, the square 106 and triangular 108 gridpatterns can alternate with one another (but not necessarily on aone-to-one basis) with movement along any given radius from the center110 of the fuel assembly to the outer perimeter, e.g., shroud 104.Additionally or alternatively, the fuel elements 102 can be arranged inconcentric circles, and the square and triangular grid patterns canalternate with one another (but not necessarily on a one-to-one basis)with movement around any one of the concentric circles.

As mentioned before, the fuel elements may be self-spacing. According toembodiments, the self-spacing can be a factor of the fuel rodcircumscribed diameter, independent of the fuel rod shape selected,however, other configurations are possible. According to certainembodiments, the fuel rods 102 may be any shape with twisted ribs (e.g.,a tube with ribs, squares, etc.). However, other shapes may be possible,such as circular cross-sections, regular geometric cross-sections, etc.

FIGS. 4-6 depict cross-sectional views of an embodiment of a fuelassembly 200 comprising four-lobe fuel rods 202, such as those describedin applicant's co-pending U.S. application Ser. Nos. 14/081,056 and13/695,792, the entire contents of which are incorporated herein byreference. According to a further aspect, certain fuel rod shapes suchas the four-lobe design, may be standardized for different reactors. Forexample, a fuel rod with a four-lobe shape, a circumscribed diameter of12±1 mm, and slight modifications may become standard for differentreactors such as the PWR and CANDU.

FIG. 4 depicts the fuel assembly 200 at an initial reference position,referred to herein as the initial 0° position. The initial 0° positioncan occur at any point along the fuel rods 202, and can occur at regularintervals. FIG. 5 depicts the fuel assembly 200 of FIG. 4 at the pointof 30° rotation of the fuel rod's lobes 204 (e.g., lengthwisedisplacement of 1/12 of the fuel rod swirl pitch) with respect to FIG.4. FIG. 6 depicts the fuel assembly of FIG. 4 at the point of 60°rotation of the fuel rods' lobes 204 (e.g., lengthwise displacement of ⅙of the fuel rod swirl pitch) with respect to FIG. 4. A 90° rotation ofthe lobes 204, or a lengthwise displacement of ¼ of the fuel rod swirlpitch, away from the position of FIG. 4 replicates the tentative initialposition of 0° shown in FIG. 4. In FIGS. 4-6, the eight fuel rods 202′indicate the only rods within the cross-section that do not have contactwith other fuel rods 202 or the shroud 206. At axial locations betweenthose shown in FIGS. 4, 5, and 6, there is no lengthwise contact of thefuel rods with one another or with the shroud 206. Accordingly, the fuelassembly is self-spacing and all the fuel rods are self-spaced along thelength of the assembly.

As mentioned previously, the fuel rods can comprise the four-lobe fuelrods described in applicant's co-pending U.S. application Ser. Nos.14/081,056 and 13/695,792. However, according to alternativeembodiments, any of the four-lobe fuel rods in the afore-described fuelassemblies can replaced by standard pelleted cylindrical fuel rods(uranium or thorium), or burnable poison bearing fuel rods (e.g.,containing gadolinium (Gd), erbium (Er), and/or dysprosium (Dy)).

As used throughout this application, the term “shroud” encompasses avariety of different designs that can surround the fuel rod bundle,either partially or completely. For example, according to embodiments, a“shroud” can be a solid dodecagonal shroud, perforated or with slits.Alternatively, the “shroud” can comprise individual bands or a shroudingstrip, or riveting on cylindrical shell (e.g., solid or “openwork” withslits). Moreover, the term “shroud” can encompass other similarstructures and designs apparent to one of ordinary skill in the artbased on this description.

The foregoing illustrated embodiments are provided to illustrate thestructural and functional principals of the present invention and arenot intended to be limiting. To the contrary, the principles of thepresent invention are intended to encompass any and all changes,alterations, and/or substitutions within the spirit and scope of thefollowing claims.

What is claimed is:
 1. A fuel assembly for use in an internal corestructure of a nuclear power reactor, the fuel assembly comprising: aplurality of fuel elements, each fuel element of the plurality of fuelelements having a longitudinal centerline, wherein the plurality of fuelelements are arranged into a predetermined mixed grid pattern thatincludes a first grid pattern and a second grid pattern different fromthe first grid pattern, the first grid pattern and the second patterneach being defined by the longitudinal centerlines of the plurality offuel elements, wherein the first grid pattern does not encompass thepattern of the second grid pattern and the second grid pattern does notencompass the pattern of the first grid pattern, wherein thelongitudinal centerlines of a subset of the plurality of fuel elementsare separated from the longitudinal centerlines of adjacent fuelelements by a common centerline-to-centerline distance, and acircumscribed diameter of each of the fuel elements of the subset isequal to the centerline-to-centerline distance, the circumscribeddiameter being equal to the largest cross-sectional dimension of thefuel element taken along a length of the entire respective fuel element,the length of the fuel element extending parallel to the longitudinalcenterline.
 2. The fuel assembly of claim 1, wherein the longitudinalcenterline of each of the plurality of fuel elements of the first gridpattern is separated from the longitudinal centerline of adjacent fuelelements by the common centerline-to-centerline distance, and acircumscribed diameter of each of the fuel elements of the first gridpattern is equal to the centerline-to-centerline distance.
 3. The fuelassembly of claim 2, wherein the longitudinal centerline of each of theplurality of fuel elements of the second grid pattern is separated fromthe longitudinal centerline of adjacent fuel elements by the commoncenterline-to-centerline distance, and a circumscribed diameter of eachof the fuel elements of the second grid pattern is equal to thecenterline-to-centerline distance.
 4. The fuel assembly of claim 1,wherein a fuel element located in the first grid pattern with a firstset of fuel elements is simultaneously located in the second gridpattern with a second set of fuel elements.
 5. The fuel assembly ofclaim 1, wherein the first grid pattern and the second grid patternalternate with one another.
 6. The fuel assembly of claim 1, wherein theplurality of fuel elements are arranged in concentric circles andwherein the first grid pattern and the second grid pattern alternatewith another along one or more of the concentric circles.
 7. The fuelassembly of claim 6, wherein the first grid pattern and the second gridpattern alternate along one or more of the concentric circles in athree-to-one basis.
 8. The fuel assembly of claim 1, wherein the firstgrid pattern is one of a rectangular grid pattern, an isometric gridpattern, a parallelogram grid pattern, a triangular grid pattern, and anequilateral triangular grid pattern.
 9. The fuel assembly of claim 1,wherein the first grid pattern is a rectangular grid pattern and thesecond grid pattern is a triangular grid pattern.
 10. The fuel assemblyof claim 1, wherein the plurality of fuel elements are four-lobe fuelrods, pelleted cylindrical fuel rods, or burnable poison bearing fuelrods.
 11. The fuel assembly of claim 1, wherein the plurality of fuelelements are helically twisted fuel elements having a multi-lobedprofile that includes spiral ribs.
 12. The fuel assembly of claim 1,wherein the plurality of fuel elements further comprise a perimetercircumscribing the plurality of fuel elements.
 13. The fuel assembly ofclaim 12, wherein the perimeter is a shroud.
 14. The fuel assembly ofclaim 12, wherein the plurality of fuel elements consists of 61 fuelelements and wherein the perimeter is a shroud circumscribing the 61fuel elements, the shroud having a cross-sectional shape of a circle ordodecagon.
 15. The fuel assembly of claim 12, wherein the plurality offuel elements consists of 19 fuel elements and wherein the perimeter hasa shape of a circle circumscribing the 19 fuel elements.
 16. The fuelassembly of claim 1, wherein the plurality of fuel elements occupy atleast 83% of a cross-sectional area of a circle that circumscribes thefuel assembly.